acc: support logging from acceptance tests (#3789)

## Changes
If you output to file with prefix "LOG", it gets logged to test log.

## Why
Debugging acceptance tests without messing expected output.

## Tests
New self test.

Author: denik

acceptance/README.md

@@ -18,7 +18,9 @@ For more complex tests one can also use:
 - `trace` helper: prints the arguments before executing the command.
 - custom output files: redirect output to custom file (it must start with `out`), e.g. `$CLI bundle validate > out.txt 2> out.error.txt`.
 
-See [selftest](./selftest) for a toy test.
+Any file starting with "LOG" will be logged to test log (visible with go test -v).
+
+See [selftest](./selftest) for more examples.
 
 ## Running acceptance tests on Windows
 

acceptance/acceptance_test.go

@@ -714,6 +714,17 @@ func runTest(t *testing.T,
 		if config.CompiledIgnoreObject.MatchesPath(relPath) {
 			continue
 		}
+		if strings.HasPrefix(filepath.Base(relPath), "LOG") {
+			prefix := relPath + ": "
+			messages := testutil.ReadFile(t, filepath.Join(tmpDir, relPath))
+			messages = strings.TrimRight(messages, "\r\n \t")
+			messages = prefix + strings.ReplaceAll(messages, "\n", "\n"+prefix)
+			if strings.Contains(messages, "\n") {
+				messages = "\n" + messages
+			}
+			t.Log(messages)
+			continue
+		}
 		unexpected = append(unexpected, relPath)
 		if strings.HasPrefix(relPath, "out") {
 			// We have a new file starting with "out"

acceptance/selftest/log/out.test.toml

@@ -0,0 +1,13 @@
+# If logs are visible (go test -v), this will produce this:
+#    acceptance_test.go:725:
+#        LOG: main log
+#        LOG: no replacements: 123456789
+#        LOG: Tue Oct 21 12:07:45 CEST 2025
+#    acceptance_test.go:725: subdir/LOG.extra: extra log
+
+echo "main log" >> LOG
+echo "no replacements: 123456789" >> LOG
+date >> LOG
+
+mkdir subdir
+echo "extra log" >> subdir/LOG.extra